diff --git a/src/calibre/gui2/actions/device.py b/src/calibre/gui2/actions/device.py
index 9779e05cad..f8515e7059 100644
--- a/src/calibre/gui2/actions/device.py
+++ b/src/calibre/gui2/actions/device.py
@@ -17,15 +17,23 @@ from calibre.utils.smtp import config as email_config
 
 def local_url_for_content_server():
     from calibre.srv.opts import server_config
+    from calibre.utils.network import is_ipv6_addr, get_fallback_server_addr
+
     opts = server_config()
-    interface = opts.listen_on or '0.0.0.0'
-    interface = {'0.0.0.0': '127.0.0.1', '::':'::1'}.get(interface)
+    interface = opts.listen_on or get_fallback_server_addr()
+
+    addr_map = {'0.0.0.0': '127.0.0.1',
+                '::':      '::1'}
+    if interface in addr_map:
+        interface = addr_map[interface]
+
     protocol = 'https' if opts.ssl_certfile and opts.ssl_keyfile else 'http'
     prefix = opts.url_prefix or ''
+
     port = opts.port
-    if ':' in interface:
-        interface = f'[{interface}]'
-    return f'{protocol}://{interface}:{port}{prefix}'
+    addr = f'[{interface}]' if is_ipv6_addr(interface) else f'{interface}'
+
+    return f'{protocol}://{addr}:{port}{prefix}'
 
 
 def open_in_browser():
diff --git a/src/calibre/gui2/preferences/server.py b/src/calibre/gui2/preferences/server.py
index e7c10a8883..d3812377dc 100644
--- a/src/calibre/gui2/preferences/server.py
+++ b/src/calibre/gui2/preferences/server.py
@@ -1299,10 +1299,20 @@ class ConfigWidget(ConfigWidgetBase):
         self.stopping_msg.accept()
 
     def test_server(self):
+        from calibre.utils.network import is_ipv6_addr, get_fallback_server_addr
+
         prefix = self.advanced_tab.get('url_prefix') or ''
         protocol = 'https' if self.advanced_tab.has_ssl else 'http'
-        lo = self.advanced_tab.get('listen_on') or '0.0.0.0'
-        lo = {'0.0.0.0': '127.0.0.1', '::':'::1'}.get(lo)
+        lo = self.advanced_tab.get('listen_on') or get_fallback_server_addr()
+
+        addr_map = {'0.0.0.0': '127.0.0.1',
+                    '::':      '::1'}
+        if lo in addr_map:
+            lo = addr_map[lo]
+
+        if is_ipv6_addr(lo):
+            lo = f'[{lo}]'
+
         url = '{protocol}://{interface}:{port}{prefix}'.format(
             protocol=protocol, interface=lo,
             port=self.main_tab.opt_port.value(), prefix=prefix)
diff --git a/src/calibre/srv/loop.py b/src/calibre/srv/loop.py
index c1f46f6305..61abdb35f4 100644
--- a/src/calibre/srv/loop.py
+++ b/src/calibre/srv/loop.py
@@ -29,6 +29,7 @@ from calibre.utils.localization import _
 from calibre.utils.logging import ThreadSafeLog
 from calibre.utils.mdns import get_external_ip
 from calibre.utils.monotonic import monotonic
+from calibre.utils.network import get_fallback_server_addr
 from calibre.utils.socket_inheritance import set_socket_inherit
 from polyglot.builtins import iteritems
 from polyglot.queue import Empty, Full
@@ -399,7 +400,7 @@ class ServerLoop:
         ba = (self.opts.listen_on, int(self.opts.port))
         if not ba[0]:
             # AI_PASSIVE does not work with host of '' or None
-            ba = ('0.0.0.0', ba[1])
+            ba = (get_fallback_server_addr(), ba[1])
         self.bind_address = ba
         self.bound_address = None
         self.connection_map = {}
@@ -508,17 +509,23 @@ class ServerLoop:
             self.setup_socket()
 
     def serve(self):
+        from calibre.utils.network import is_ipv6_addr
+
         self.connection_map = {}
         if not self.socket_was_preactivated:
             self.socket.listen(min(socket.SOMAXCONN, 128))
         self.bound_address = ba = self.socket.getsockname()
         if isinstance(ba, tuple):
-            ba = ':'.join(map(str, ba))
+            if is_ipv6_addr(ba[0]):
+                addr = f'[{ba[0]}]'
+            else:
+                addr = f'{ba[0]}'
+            ba_str = f'{addr}:' + ':'.join(map(str, ba[1:]))
         self.pool.start()
         with TemporaryDirectory(prefix='srv-') as tdir:
             self.tdir = tdir
             if self.LISTENING_MSG:
-                self.log(self.LISTENING_MSG, ba)
+                self.log(self.LISTENING_MSG, ba_str)
             self.plugin_pool.start()
             self.ready = True
 
diff --git a/src/calibre/srv/opts.py b/src/calibre/srv/opts.py
index b9ebffe6e7..f6e3fd7e27 100644
--- a/src/calibre/srv/opts.py
+++ b/src/calibre/srv/opts.py
@@ -110,8 +110,8 @@ raw_options = (
     ' there are more than this number of items. Set to zero to disable.'),
 
     _('The interface on which to listen for connections'),
-    'listen_on', '0.0.0.0',
-    _('The default is to listen on all available IPv4 interfaces. You can change this to, for'
+    'listen_on', None,
+    _('The default is to listen on all available IPv6 and IPv4 interfaces. You can change this to, for'
     ' example, "127.0.0.1" to only listen for connections from the local machine, or'
     ' to "::" to listen to all incoming IPv6 and IPv4 connections.'),
 
diff --git a/src/calibre/utils/mdns.py b/src/calibre/utils/mdns.py
index 3e401e9572..f2fbe1a190 100644
--- a/src/calibre/utils/mdns.py
+++ b/src/calibre/utils/mdns.py
@@ -77,6 +77,9 @@ def _get_external_ip():
 
 
 def verify_ipV4_address(ip_address):
+    if ip_address == None:
+        return None
+
     result = None
     if ip_address != '0.0.0.0' and ip_address != '::':
         # do some more sanity checks on the address
diff --git a/src/calibre/utils/network.py b/src/calibre/utils/network.py
index 410206df81..e2d071f594 100644
--- a/src/calibre/utils/network.py
+++ b/src/calibre/utils/network.py
@@ -108,3 +108,18 @@ def internet_connected():
         DummyNetworkStatus()
 
     return internet_connected.checker()
+
+def is_ipv6_addr(addr):
+    import socket
+    try:
+        socket.inet_pton(socket.AF_INET6, addr)
+        return True
+    except OSError:
+        return False
+
+def get_fallback_server_addr():
+    from socket import has_dualstack_ipv6
+    if has_dualstack_ipv6():
+        return '::'
+    else:
+        return '0.0.0.0'